diff options
Diffstat (limited to 'subx')
| -rw-r--r-- | subx/029translate.cc | 65 |
1 files changed, 62 insertions, 3 deletions
diff --git a/subx/029translate.cc b/subx/029translate.cc index a813cd4d..166762d9 100644 --- a/subx/029translate.cc +++ b/subx/029translate.cc @@ -2,9 +2,6 @@ //: building levels above it that make programming in x86 machine code a //: little more ergonomic. //: -//: Higher levels will be in later layers. Since we can stop at any layer, we -//: can execute levels from bedrock up to any level. -//: //: All levels will be "pass through by default". Whatever they don't //: understand they will silently pass through to lower levels. //: @@ -19,6 +16,68 @@ //: //: Higher levels usually transform code on the basis of metadata. +//: Ordering transforms is a well-known hard problem when building compilers. +//: In our case we also have the additional notion of layers. The ordering of +//: layers can have nothing in common with the ordering of transforms when +//: SubX is tangled and run. This can be confusing for readers, particularly +//: if later layers start inserting transforms at arbitrary points between +//: transforms introduced earlier. Over time adding transforms can get harder +//: and harder, having to meet the constraints of everything that's come +//: before. It's worth thinking about organization up-front so the ordering is +//: easy to hold in our heads, and it's obvious where to add a new transform. +//: Some constraints: +//: +//: 1. Layers force us to build SubX bottom-up; since we want to be able to +//: build and run SubX after stopping loading at any layer, the overall +//: organization has to be to introduce primitives before we start using +//: them. +//: +//: 2. Transforms usually need to be run top-down, converting high-level +//: representations to low-level ones so that low-level layers can be +//: oblivious to them. +//: +//: 3. When running we'd often like new representations to be checked before +//: they are transformed away. The whole reason for new representations is +//: often to add new kinds of automatic checking for our machine code +//: programs. +//: +//: Putting these constraints together, we'll use the following broad +//: organization: +//: +//: a) We'll divide up our transforms into "levels", each level consisting +//: of multiple transforms, and dealing in some new set of representational +//: ideas. Levels will be added in reverse order to the one their transforms +//: will be run in. +//: +//: To run all transforms: +//: Load transforms for level n +//: Load transforms for level n-1 +//: ... +//: Load transforms for level 2 +//: Run code at level 1 +//: +//: b) *Within* a level we'll usually introduce transforms in the order +//: they're run in. +//: +//: To run transforms for level n: +//: Perform transform of layer l +//: Perform transform of layer l+1 +//: ... +//: +//: c) Within a level it's often most natural to introduce a new +//: representation by showing how it's transformed to the level below. To +//: make such exceptions more obvious checks usually won't be first-class +//: transforms; instead code that keeps the program unmodified will run +//: within transforms before they mutate the program. +//: +//: Level l transforms programs +//: Level l+1 inserts checks to run *before* the transform of level l runs +//: +//: This may all seem abstract, but will hopefully make sense over time. The +//: goals are basically to always have a working program after any layer, to +//: have the order of layers make narrative sense, and to order transforms +//: correctly at runtime. + :(before "End Main") if (is_equal(argv[1], "translate")) { START_TRACING_UNTIL_END_OF_SCOPE; |